1. Field of the Invention
The present invention relates to an equalizing circuit and method for communication terminal equipment, and more particularly, to an equalizing circuit and method for compensating for the attenuation and distortion of signals received through a public telephone network.
2. Description of the Related Art
In general, data communication terminal equipment includes a key phone system and a facsimile system, etc., and among the rest, the facsimile system is used for transmitting/receiving a document. In this facsimile system, the transmission and reception of image data of the document are performed according to a process of a protocol after a document is inserted in the document feeding cassette and a receiving party's telephone number is then dialed. The protocol for transmitting and receiving the document is prescribed in a CCITT recommendation, and the protocol in accordance with CCITT.41420 is performed through five phases as shown in the following Table &lt;1&gt;.
TABLE 1 __________________________________________________________________________ CALLING TRANSMITTER CALLED RECEIVER __________________________________________________________________________ PHASE A CALLING(CNG) CALLED STATION IDENTIFICATION(CED) PHASE B COMMAND CAPABILITIES IDENTIFICATION INFORMATION (NSF + CSI + DIS) (NSS + TSI + DCS) RECEPTION CONFIRMATION TRAINING (HIGH SPEED) (CFR) (9600/7200/4800/2400) PHASE C MESSAGE HIGH SPEED (9600/7200/4800/2400) PHASE D END OF MESSAGE RECEPTION CONFIRMATION (EOP/MPS/EOM/RPI-Q) (MCF/RTP/RTN/PIP/PIN) PHASE E DISCONNECT THE LINE (DCN) __________________________________________________________________________
As shown in Table &lt;1&gt;, in phase A, which is a call setup phase, the calling transmitter and the called receiver exchange the CNG and CED signal with each other. In phase B, which is a premessage procedure phase, the calling transmitter and the called receiver exchange the NSS+TSI+DCS and NSF+CSI+DIS and CFR signals with each other. The phase C is a phase for transmitting a fax message (image data). Furthermore, the phase D is a postmessage procedure phase and in this phase, the calling transmitter and the called receiver exchange the EOP/MPS/EOM/RPI-Q and the MCF/RTP/RTNIP/PIN with each other, and the phase E is a phase of transmitting the DCN signal and releasing the call.
However, in the facsimile system as described above, as a cable length of the public telephone network is extended, signals transmitted through this cable are attenuated as shown in FIG. 1A. Particularly, signals of a high frequency are attenuated more than signals of a low frequency due to a capacitance factor of the cable as shown in FIG. 1B. Accordingly, in order to compensate for the attenuation of the signals as mentioned above, data communication terminal equipment is generally provided with an equalization function. The aforesaid equalization function is an optional function and therefore, the equalization function appropriate to the status set by the user is effected. For example, when the user sets a cable length that is one of the optional items in the facsimile system as 0.0 Km, the equalization function is not effected. On the other hand, if the user sets the cable length as 1.8 Km or 3.6 Km, the equalization function appropriate to this cable length is effected. As discussed previously, in the conventional data communication terminal equipment, the user has to set the equalization function appropriate to the corresponding cable length in a setup menu and therefore, there is a problem in that in case that the equalization function inappropriate to the corresponding cable length is set or the set equalization function is not designated in setup menu, the communication is not completely established.
Ukegawa and Iizuka, et al., U.S. Pat. Nos. 5,150,400 and 4,736,249 each disclose facsimile apparatus which include training procedures prior to the normal transmission of documents. However, neither of these references teach or suggest the use of the handshake process signals to determine the equalization necessary for the telephone line as in the present invention.